The present invention relates to a wireless communication method and apparatus, and, more particularly, to a method for avoiding Packet Data Convergence Protocol (PDCP) downlink Service Data Unit (SDU) duplication in eNodeB under Radio Link Control acknowledged mode (RLC-AM).
FIG. 1 shows a wireless communication network 100 including a first cell 102 and a second cell 104, which are coverage areas of a first eNodeB 106 and a second eNodeB 108 respectively. An eNodeB (Evolved Node B) is the hardware that is connected to a mobile phone network that communicates directly with mobile handsets, like a base transceiver station. A user equipment (UE) 110 in the first cell 102 is receiving downlink information through a link 112 from the first eNodeB 106.
FIG. 2 is a diagram of a Layer-2 200 used in current wireless communication networks, for instance an LTE (Long Term Evolution) network. In a downlink transmission, Layer-2 200 is split into the following sublayers: Medium Access Control (MAC) sublayer 202, RLC (Radio Link Control) sublayer 204 and PDCP sublayer 206. A PDCP-RLC interface 208 connects the PDCP sublayer 206 to the RLC sublayer 204 for submitting data packets created in the PDCP sublayer 206 to the RLC sublayer 204 when the data packet is to be transmitted from the first eNodeB 106 to the UE 110. When the UE 110 is receiving downlink data from the eNodeB 106, e.g., streaming videos, downloading files, running games or surfing web pages online, Data Radio Bearers (DRBs) 210 created in the PDCP layer 206 for transfer of user data between the UE 110 and the first eNodeB 106 are RLC-AM mapped, which means every data packet transmitted from the eNodeB 106 to the UE 110 should be acknowledged with a signal through a link 114 to ensure lossless handover when the UE 110 is moving from the first cell 102 to the second cell 104. The first eNodeB 106 stores and transmits un-acknowledged data through the link 116 to the second eNodeB 108, therefore the second eNodeB 108 can continue to transmit un-acknowledged data over the link 118 to the UE 110 after the UE 110 enters the second cell 104.
In current eNodeBs, to ensure lossless handover, original PDCP DL SDUs are duplicated and listed in a re-transmission queue in the eNodeB with their addresses in a buffer. The duplicated SDUs are transformed to PDUs and the PDUs are transmitted to the UE. The original SDUs are maintained at their original addresses. When a handover occurs, the original SDUs listed in the re-transmission queue are sent to a target eNodeB.
The duplication of PDCP DL SDUs for re-transmission in case of handover uses alot of CPU cycles, which can cause a 20% performance drop when compared to the performance in unacknowledged mode (UM) where there is no SDU duplication. Further, the original SDUs are discarded either when they are acknowledged or when acknowledgement is not received within a predetermined time period. The cycles required for duplicating PDCP DL SDUs are mostly wasted because handover doesn't occur frequently when compared to DL data rate. It is therefore desirable to avoid PDCP DL SDU duplication in eNodeB under RLC-AM mode.